The invention relates generally to the field of image capture, and more specifically to a method of compressing an image from an extended dynamic range image sensing device.
Image sensing devices, such as a charge-coupled device (CCD), are commonly found in such products as digital cameras, scanners, and video cameras. These image sensing devices have a limited dynamic range when compared to traditional photographic film products. A typical electronic image sensing device has a dynamic range of about 7 stops. This means that the exposure for a typical scene must be determined with a fair amount of accuracy in order to avoid clipping the resultant signal By contrast, natural scenes often exhibit a dynamic range of 9 stops and higher. This is mainly a consequence of multiple light sources with widely varying intensities illuminating the scene objects. Specular highlights also contribute to the dynamic range of natural scenes.
Electronic sensors used to scan photographic film must also contend with a high dynamic range of signal intensities. In U.S. Pat. No. 5,221,848 issued Jun. 22, 1993 to Milch entitled High Dynamic Range Film Digitizer and Method of Operating the Same discloses a method and apparatus designed to extend the dynamic range of an electronic image sensor. Aimed primarily for scanning photographic film, Milch teaches a method of a one pass film scanner using a charge-coupled device scanner having a plurality of linear arrays thereon. One of the arrays is responsive to high intensities of light and the other array is responsive to low intensities of light. The information from the two arrays is then combined and digitized forming an extended dynamic range digital image. The method and apparatus disclosed by Milch is an electronic image sensor having photosites with the same spectral sensitivity but different inherent response to intensities of light which is capable of producing digital images with very high dynamic range.
Digital electronic cameras employ a single image sensor with a color filter array (CFA) to produce a sparsely sampled digital image. A typical color filter array pattern which utilizes red, green, and blue photosites for producing red, green, and blue pixel values is disclosed in U.S. Pat. No. 3,971,065, issued Jul. 20, 1976 to Bayer entitled Color Imaging Array. Interpolation algorithms are employed to produce a full resolution color image from the sparsely sampled image. Digital cameras also need to record scenes having a high dynamic range. One way to obtain a high dynamic range image from a digital camera is to employ a high bit depth analog to digital converter in the camera. Another way is to employ an image sensor having interspersed fast and slow photosites as disclosed in copending U.S. Ser. No. 09/615,398 filed Jul. 13, 2000 by Gallagher et al., which is incorporated herein by reference. Also an important feature of digital cameras is the need to produce digital images of manageable size. This is primarily due to the limited on board memory of the digital camera and due to the limitations in bandwidth for transferring the digital images to other devices.
Robert Gray and Bobby Hunt quantify an advantage associated with performing an image compression separately on the red, green, and blue pixel values of an image captured by an image sensor having a CFA in the article xe2x80x9cMultispectral data compression using staggered detector arrays,xe2x80x9d Proc. SPIE, Volume. 432, San Diego, August 1983. However, the method of Gray and Hunt did not consider image sensors having fast pixel values derived from fast photosites with a predetermined response to a light exposure, and slow pixel values derived from slow photosites with a slower response to the same light exposure.
Commonly assigned U.S. Pat. No. 5,065,229 issued Nov. 12, 1991 and U.S. Pat. No. 5,053,861 issued Oct. 1, 1991, both issued to Tsai et al. and both entitled Compression Method and Apparatus for Single-Sensor Color Imaging Systems describe a method of compressing digital images prior to color interpolation. However, their method does not consider image sensors having fast pixel values derived from fast photosites with a predetermined response to a light exposure and slow pixel values derived from slow photosites with a slower response to the same light exposure.
Therefore, there exists a need to efficiently and effectively compress a digital image originating from an electronic image sensor having fast pixel values derived from fast photosites with a predetermined response to a light exposure and slow pixel values derived from slow photosites with a slower response to the same light exposure. Furthermore, there is a need to efficiently compress images created with an electronic image sensor having photosites with the same spectral sensitivity but different inherent response to intensities of light which is capable of producing digital images with very high dynamic range.
The need is met according to the present invention by providing a method of producing a compressed image from a full resolution sparsely sampled extended dynamic range image produced by a sparsely sampled extended dynamic range image sensing device having fast photosites with a predetermined response to a light exposure for producing fast pixel values, and slow photosites with a slower response to the same light exposure for producing slow pixel values, that includes the steps of: providing a full resolution sparsely sampled extended dynamic range image; separating the full resolution sparsely sampled image into a fast image comprising only fast pixel values and a slow image comprising only slow pixel values; and separately compressing the fast image and the slow image.
The advantage of the present invention is that the compression technique described herein explicitly takes advantage of the unique arrangement of photosites on an imaging sensor having fast photosites with a predetermined response to a light exposure and slow pixel values derived from slow photosites with a slower response to the same light exposure in order to more efficiently compress a digital image from such a sensor.